The present invention relates to a fuel evaporator to be installed in an intake manifold of an internal combustion engine, which comprises a heating element made of ceramic material having positive temperature coefficient of resistance, particularly to a supporting structure for the heating element of the fuel evaporator.
Ceramic material such as barium titanate (hereinafter will be called PTC ceramic) has such characteristics that when electrified, it generates heat and when the temperature thereof reaches a specific value (called Curie temperature), the electric resistance thereof increases rapidly. Under the Curie temperature, the PTC ceramic receives a large amount of electric current to reach the Curie temperature rapidly. Over the Curie temperature, the electric resistance becomes remarkably large and the PTC ceramic receives only a small amount of electric current, so that the PTC ceramic is not overheated. Thus, the PTC ceramic adjusts the temperature thereof by itself into about the Curie temperature.
Therefore, by using the PTC ceramic as the heating element for promoting the evaporation of the fuel contained within the intake mixture supplied into the internal combustion engine, the intake mixture can be heated at a moment even at a cold starting time.
Thus, according to this device, the fuel within the intake mixture can be effectively evaporated compared with other heating means such as exhaust gas and engine coolant.
In the conventional fuel evaporator using the PTC ceramic as a heating element, a casing made of a heat conductive metallic plate is disposed in one portion of the intake manifold directly under the throttle valve. And one portion of the casing is exposed to the interior of the intake manifold. Within the casing, the heating element is closely fixed to the under surface of the exposed portion of the casing.
One of the problems of the above described conventional fuel evaporator is that heat generated by the heating element escapes to the wall of the intake manifold through the metallic casing to lower the heating efficiency.
In order to overcome the above described problems, we, inventors have proposed the structure that the side wall of the casing is covered with the electric insulating material such as synthetic resin and that the casing is fixed to the wall of the intake manifold through the cover member. (Japanese Utility Model Applications No. 113959/1979 and No. 64721/1980, which are the priority cases for our copending application Ser. No. 177,799 filed Aug. 13, 1980.)
Another problem of the above described conventional fuel evaporator is that it is very difficult to closely fix the fragile heating element made of ceramic to the casing without being broken.
In general, the heating element is closely fixed to the casing by means of a wave member, a coil spring or the like.
However, since the wave washer does not spring so largely so as to compensate the vibration of the device, the load applied to the heating element becomes ununiform. Consequently, the heating element is in danger of slipping relative to the casing.
And when the coil spring is used, particularly higher load is applied to the connecting portions of the heating element with the coil spring, compared with the other portions thereof.
As a result, the heating element is in danger of being broken.
In order to make the pressing force of the coil spring applied to the heating element uniform, a metallic plate can be interposed between the heating element and the coil spring.
However, in this case, it is very difficult to machine the opposed surfaces of the metallic plate and the heating element so as to completely fit to each other.
When the load of the coil spring is applied to the portion of the metallic plate with which the heating element is partially contacted, the heating element is apt to be broken in the portion.
Accordingly, one object of the present invention is to provide a fuel evaporator comprising a heating element made of PTC ceramic, and having high heating efficiency and excellent durability.
Another object of the present invention is to provide a supporting structure for uniformly and closely fixing the heating element made of PTC ceramic to the fuel evaporator without being broken.